All major steelmaking processes require the input of iron bearing materials as process feedstocks. If the steelmaking method uses a basic oxygen furnace, the iron bearing materials are usually blast furnace hot metal and steel scrap. To augment scrap supplies in times of high demand or to provide a more pure product, steelmakers seek and use alternative iron sources in addition to conventional hot metal and scrap. The most well known and broadly used alternative iron source is a product known as Direct Reduced Iron ("DRI") which is produced by the solid state reduction of iron ore without the formation of liquid iron.
Direct reduction of iron oxides captured steelmakers' imaginations several centuries ago when they first realized how easily oxygen could be removed from its iron ore carrier through reduction with hydrogen and/or carbon monoxide. However, harnessing the simple chemical reactions in large scale commercial production proved elusive. Then, in the early 1970's, the Midrex direct reduction process was developed. In the Midrex direct reduction process, reduction of iron ore oxides to iron is accomplished by forming a bed of iron containing burden, such as iron ore pellets, in a shaft furnace then injecting a reduction gas, typically a mixture of hydrogen and carbon monoxide, into the burden at temperatures less than 1000.degree. C. The reduction gas is typically injected into the burden using a bustle and tuyere system.
In some facilities the throughput of direct reduction furnaces using the Midrex process has increased to such extent that the furnaces currently operate at twice their original capacity. In high capacity plants, where throughput of burden can reach 13-14 tons per cubic meter of furnace volume, the volume of gas necessary to operate the furnace has greatly increased over original designs and the velocity of gas leaving the tuyeres can reach 130 m/s. In conducting research into the operation of direct reduction furnaces operating under such conditions it was discovered that at elevated gas volumes and velocities, both mechanical and chemical problems are encountered.
High gas velocities can cause abrasion of the tuyeres. High gas volume velocity also tends to push the burden to the center of the furnace which disturbs the pressure distribution inside the furnace and creates a "funnel flow" or "slump" in the burden. Stated differently, high gas volumes and velocity creates a "bubble" in the descending burden at the tuyeres. The non-uniform flow of burden in the furnace allows reduction gas to escape unreacted by traveling up the sides of furnace. When unreacted gas escapes up the sides of the furnace, the center of the furnace tends to operate cooler than the perimeter thereby reducing the efficiency of the reduction process. Finally, non-uniform flow of burden results in unreduced or partially reduced burden leaving the reactor.
Simply put, the direct reduction of iron oxide has evolved beyond the limits of the equipment designed to carry out the direct reduction reaction. Therefore, a need exists for an improved system to inject reduction gas into a direct reduction furnace.